1. Field of the Invention
The present invention relates to a flowmeter for measuring the flow rate of fluids, and in particular to a flowmeter for obtaining a flow rate by detecting the rotational speed of impellers which are caused to rotate by fluid.
2. Description of the Related Art
There are generally several types of flowmeters for detecting the flow rate of fluids, for example, a type that detects the calorie taken away from by movements of the fluid, and a type that detects mechanical movements activated by a force which the fluid has, etc. However, either type has shortcomings because large errors are produced in the detected flow rate or they may influence the flow of the fluid by the detection operation itself which creates a resistance to the flow of the fluid, etc.
An impeller type flowmeter, shown in FIG. 1, represented by a waterway flowmeter, has such a mechanism that a flow channel 1b of fluid is produced in the direction of the arrow A--A which is the tangential direction of an impeller 1a, wherein the impeller 1a rotates due to the force of the flowing fluid when the flow exceeds a certain flow rate, and rotation of the impeller 1a is detected mainly by a gear unit (not illustrated) in the vicinity of the axis 1c of the impeller 1a which is the center of rotation, and the detected rotation is transmitted to an instrument panel where the flow rate of the fluid is displayed.
Furthermore, although the fuel injection quantity must be accurately controlled in a diesel engine, the injection quantity has conventionally been detected indirectly by calculating the fuel injection quantity from the number of engine rotations at that time through detecting the control rack position of an in-line fuel injection pump or detecting the control sleeve position of a distributor type fuel injection pump.
There are some problems associated with this type of detection such as wear of and backlashes in a gear mechanism. It was difficult to make an impeller type flowmeter very small and to accurately detect the flow rate of high speed fluids due to a complicated construction of a mechanism which consists of mechanical elements.
Furthermore, in a fuel injection pump of a diesel engine, it is difficult to accurately detect the fuel injection quantity because the injection quantity may change due to inconsistencies in the quality of such factors as machining accuracy, assembling accuracy, etc., and chronological changes thereof.